Biomedical Engineering Reference
In-Depth Information
self-motion
com
p
onent
(a)
optic flow field
(b)
object-motion component
(c)
Fig. 4.5 a
Optic flow field generated by combined motion of observer and object (
black dot
).
b
The component of optic flow due to self-motion independent of object motion.
c
The component
of optic flow due to object motion independent of self-motion. The optic flow field (
a
) is the vector
sum of the self-motion (
b
) and object-motion (
c
) components
the vector sum of the object-motion component and the self-motion component (see
Fig.
4.5
). Information about
ν
min
is found in the object-motion component of optic
flow. This is because the optical specification of
˙
z
m
/
E
involves
γ
m
(see Fig.
4.4
b),
which is the component of
that is due to the motion of the object independent of
the observer's self-motion. Recall that
γ
is the visual angle between eye level and
the base of the moving object (see Fig.
4.1
).
γ
γ
, which is the rate of change of
γ
,is
γ
influenced by the movement of both the observer and the object. Specifically,
is
γ
o
(the rate of change of
γ
γ
m
the sum of
due to the observer's self-motion) and
(the rate of change of
γ
due to object motion). Because the optical specification
of
ν
min
while moving requires the
visual system to factor out the influence of self-motion.
In principle, factoring out the self-motion component of optic flow could be
achieved using visual information about self-motion, non-visual information about
self-motion, or some combination of both. This leads to the prediction that manip-
ulations of visual and/or non-visual self-motion information should influence the
detection of information about
˙
z
m
/
E
involves
γ
m
, detecting information about
ν
min
and any actions that are selected on the basis
of such information. This prediction was recently tested using a VE to manipulate
self-motion information (Fajen and Matthis, in press; [
8
]).
4.6 Testing the Affordance-Based Approach
The task required subjects to judge whether they could safely walk through a shrink-
ing gap between a pair of converging obstacles before the gap closed. On each trial,
subjects were walking along a path at steady state when two cylindrical obstacles in
the VE positioned symmetrically about the path began to converge toward a point
